1. Field of the Invention
The present invention relates to a wavelength converter capable of generating a wavelength tunable laser optical source in itself without any separate wavelength tunable laser and using the generated optical source as a probe optical source of the wavelength converter.
2. Description of the Related Art
Recently, with a rapid increase of the amount of information at home and abroad, the capacity of a transmission system has become massive. Particularly, a wavelength division multiplexing (WDM) system that can efficiently use wide bandwidths provided by optical fiber using optical wavelengths of various channels becomes a matter of great concern. The core element in a communication network of a transmission system using such a WDM system is an optical wavelength converter in addition to an optical amplifier.
The optical wavelength converter is a device for converting the wavelength of a transmitted signal irrespective of a transmission speed or a transmission type, and plays the following role. First, it reduces blocking due to a wavelength contention in a WDM communication network, and makes it possible to usefully reuse the wavelength. Second, it increases the flexibility and capacity of a network with respect to fixed wavelengths. Third, it makes it possible to distribute and manage the network and enables protection switching to be performed more easily.
Schemes for implementing the optical wavelength converter that plays the above-described role using a semiconductor optical amplifier (hereinafter referred to as an “SOA”) have mainly been researched. First is an XGM (Cross Gain Modulation) system that uses an XGM characteristic in the SOA and that is implemented most simply. Second is an XPM (Cross Phase Modulation) system that uses an XPM characteristic in the SOA.
Since the XPM system has a relatively simple structure and has a superior wavelength conversion performance with respect to a high-speed data, it is used in diverse fields.
FIG. 1 is a schematic view of a Mach-Zehnder interferometer-type SOA-XPM wavelength converter. Referring to FIG. 1, the operation of the Mach-Zehnder interferometer-type SOA-XPM wavelength converter will now be explained. First, if a pump optical signal having a wavelength of λS is inputted to SOA1, the carrier density in an SOA1 active layer is decreased due to a stimulated emission. Accordingly, the index of refraction of the active layer is changed, and thus if the probe signal having a wavelength of λC passes through the SOA1, a phase change occurs. Accordingly, if an output pulse signal outputted from the SOA1 and a CW (Continuous Wave) signal outputted from SOA2 are added together at a Mach-Zehnder interferometer-type output terminal and the two signals are in an out-of-phase condition, a destructive interference occurs between the signals and no signal is outputted, while if the two signals are in an in-phase condition, a constructive interference occurs between the signals and a signal is outputted. At this time, information of the pump optical signal having a wavelength of λS is transferred to the probe signal having a wavelength of λC to cause a wavelength conversion to occur.
In the Mach-Zehnder interferometer-type SOA-XPM wavelength converter as described above, the CW probe optical signal having a wavelength of λC of which the wavelength conversion should be performed is used as an input of the wavelength converter together with the pump optical signal having a wavelength of λS which carries digital data information. Generally, the probe optical signal is provided using a separate optical source outside the wavelength converter, and this causes the whole size of the wavelength converter to increase. Also, in consideration of the case in which diverse wavelengths are subject to wavelength conversion, a continuously or discretely wavelength-tunable optical source is required. Additionally, diverse researches for integrating the probe optical source and the wavelength converter in consideration of the above-described problem have been made.
FIG. 2 is a schematic view of a wavelength converter into which a DFB (Distributed Feed-Back) laser is integrated. Referring to FIG. 2, the wavelength converter into which the DFB laser is integrated is provided by integrating the DFB laser onto the same semiconductor substrate as the probe optical source of the Mach-Zehnder interferometer-type SOA-XPM wavelength converter as illustrated in FIG. 1. However, the maximum wavelength tunable range of the DFB laser is generally limited to about 2 nm. In order to improve this, a treatise on a case that a wavelength tunable laser is integrated into a wavelength converter as a probe optical source that is operable in a wide wavelength range has recently been published.
FIG. 3 is a schematic view of an SOA-XPM wavelength converter into which a wavelength tunable laser is integrated. Referring to FIG. 3, the tunable laser 40 includes an optical gain medium 10 for providing an optical gain, a phase shift medium 20 for adjusting an optical phase and first and second distributed Bragg reflectors (DBRs) 30a and 30b operating as optical mirrors or reflectors. The first distributed Bragg reflector 30a is located in the front of the optical gain medium 10, and the second distributed Bragg reflector 30b is located in the rear of the optical gain medium 10.
By applying current to the first and second distributed Bragg reflectors 30a and 30b and the phase shift medium 20, the laser 40 is controlled to oscillate at a desired wavelength. The first and second distributed Bragg reflectors 30a and 30b perform a coarse tuning so that the tunable laser 40 oscillates at a desired wavelength. In addition, the phase shift medium 20 performs a fine tuning. By applying current from an outside source to the optical gain medium 10, the strength of the output optical source of the tunable laser 40 is controlled. The tunable laser 40 is operable even in the wavelength range of 30 nm or more ([IEEE Photonics Technology Letters, vol. 15, no. 8, 2003], [IEEE Photonics Technology Letters, vol. 16, no. 10, 2004]).
However, since the wavelength converter into which the tunable laser is integrated as illustrated in FIG. 3 is obtained by integrating the tunable laser together with the wavelength converter, it is difficult to miniaturize the wavelength converter and its power consumption becomes great.